We have statistics for traffic in London that go back nearly 90 years: the Thames Screenline traffic count is taken every two years on every bridge across the Thames. The survey was done by the Metropolitan police originally, then the Ministry forTransport and from 2000 by TfL. It counts the number of vehicles, including bikes, on every bridge across the Thames.
Recently, as well as counts taken back to the 1980s, some data came to light from a couple of years in the 1930s: https://alexinthecities.co.uk/2025/05/cycling-in-london-changing-patterns-across-the-thames/
If we combine these datasets we can see how the number of cyclists crossing the Thames has changed back to the 1935. Far from in decline, cycling in London is increasing very rapidly, and is at historically unprecedented levels. 